1. Field of the Invention
The present invention relates to an audio reproducing device and method, audio amplifier, integrated circuit for the audio amplifier, and is preferably used particularly for a digital input type digital amplifier that reproduces digital audio data recorded in a digital signal recording medium such as a CD (compact disk), MD (mini disk), DVD (digital video disk), SACD (super audio CD) or analog input type digital amplifier that amplifies an inputted analog audio signal and outputs it as an analog signal.
2. Description of the Related Art
While a conventional A-class/AB-class amplifier is called an “analog amplifier,” a D-class amplifier is called a “digital amplifier” because it has a feature of causing a power MOSFET to perform switching operation to drive a speaker. The digital amplifier has better power efficiency than a conventional analog amplifier. Thus, against a background of demands for miniaturization and low power consumption of audio devices in recent years, a growing number of audio devices use digital amplifiers.
A digital amplifier uses mainly two systems; PWM (Pulse Width Modulation) system and ΔΣ (delta sigma) system. The PWM system is a system that compares the amplitude of an analog audio signal with that of a triangular waveform, generates a pulse width modulated PWM signal and uses the PWM signal to switch a power MOSFET. On the other hand, the ΔΣ system is a system that has developed a Σ modulation, which is one of A/D conversion systems, integrates the inputted audio signal, quantizes its result and uses the resulting ΔΣ modulated signal to switch the power MOSFET. Compared to the PWM system, most part of which is constructed of analog circuits, the ΔΣ system is characterized by having fewer factors of deterioration of sound quality such as noise and distortion.
A PCM multi-bit system (hereinafter abbreviated as “PCM system”) has been conventionally used as means for expressing audio information which is originally an analog signal using a digital signal. CDs which are widely used today also adopt this PCM system. The PCM system performs calculation at every timing of a sampling frequency (44.1 kHz) according to a quantization characteristic, replaces an analog signal by a digital signal and records the absolute amount of data at all sampling points in a CD.
On the other hand, a 1-bit system is a focus of attention recently which improves recoverability from a digital signal to the original analog signal by controlling a quantization noise distribution using ΔΣ modulation compared with the PCM system. The 1-bit system only records a variation from immediately preceding data as a binary signal and uses no thinning or interpolation of an amount of information as in the PCM system, and therefore the 1-bit signal resulting from quantization has a characteristic extremely close to an analog signal.
Therefore, unlike the PCM system, an audio reproducing device (digital power amplifier) based on a 1-bit system, a so-called 1-bit amplifier, needs no D/A converter and has the merit of being capable of reproducing the original analog signal through a simple process of only removing a digital signal of a high frequency component using a low pass filter at the final stage.
FIG. 1 is a block diagram schematically showing a configuration of a conventional 1-bit amplifier using a ΔΣ system. In FIG. 1, a ΔΣ modulation processing unit 112 carries out conversion processing based on ΔΣ modulation on a multi-bit digital audio signal which is reproduced from a CD 111 to obtain a ΔΣ modulated signal (1-bit signal). Then, it outputs the ΔΣ modulated signal obtained as a control signal to drive a power switch (power amplifier) 113.
The power switch 113 is constructed of, for example, a full-bridge switching circuit, controls the time during which each switching element is in an ON state and thereby amplifies the audio signal based on a constant power supply voltage supplied to the power switch 113 and outputs it. The audio signal amplified by this power switch 113 is converted to an analog audio signal through a low pass filter (LPF) 114 and output from a speaker 115.
The digital amplifier shown in this FIG. 1 is of a digital input type, but an analog input type is also constructed roughly in the same way as this.
As stated above, using the 1-bit amplifier in such a configuration can reproduce the original analog signal without carrying out a D/A conversion operation during reproduction with a simple process by only removing a high frequency signal through the low pass filter 114.
However, such a configuration causes the power supply voltage of the power switch 113 which should originally remain at a constant level to fluctuate based on various factors, producing errors and distortion in the amplified audio signal, causing adverse effects on the sound quality of a reproduced sound.
For example, when loud sound is output, quite a high current flows into the output impedance of the power supply and the power supply voltage reduces. When the power supply voltage is reduced, the output level of the audio signal reaches the ceiling, is clipped and its waveform is distorted. Furthermore, even when a relatively small sound is output, if a sharp rising or falling audio signal is output, the power supply voltage falls or rises, causing distortion in the output waveform.
Furthermore, the MOSFET used for the power switch 113 is driven at relatively low impedance. Therefore, as shown in FIG. 2, a slowdown (dotted line) occurs in the switching characteristic at a relatively low speed, which is different from an ideal switching characteristic (solid line). However, the calculation unit of the ΔΣ modulation processing unit 112 performs quantization assuming that the switching characteristic of the power switch 113 is ideal. Therefore, if the switching characteristic has a slowdown, it is not possible to generate a voltage at a precise level, producing distortion in the output waveform. Especially when large output power is obtained, the distortion increases considerably.
Thus, to solve such a problem, a digital amplifier is provided adapted to cause the output signal of the power switch 113 producing variation in the power supply voltage or slowdown of the switching characteristic to be fed back to the ΔΣ modulation processing unit 112 and correct the variation in the power supply voltage or deviation from the ideal switching characteristic, etc., using this feedback signal in real time.
By applying similar feedback control not only to a ΔΣ system digital amplifier but also to a PWM system digital amplifier, the pulse width of a PWM signal is corrected by a feedback signal.
However, when the signal is fed back from the output stage of the power switch 113, there is a problem that under the influence of transient state variations of the MOSFET that carries out switching, corrections are not performed successfully, producing distortion in the output waveform.
The present invention has been implemented to solve such problems and it is an object of the present invention to make it possible not only to suppress deterioration of sound quality of reproduced sound caused by variations in the power supply voltage or deviation from an ideal switching characteristic of the power switch but also to suppress deterioration of sound quality of reproduced sound caused by the switching operation itself of the power switch.